The available space for an LED driver has limited many retrofit applications for LED lamps, such as a tubular LED lamp. The traditional LED architecture and driver topology for tubular LEDs requires a driver space much larger than the space that the existing tube dimensions (e.g. a T5 tube) can offer. A tapped linear driver design is therefore considered as a good alternative solution for applications with space constraints, since it requires much smaller power components and enables the driver size to be minimized.
A problem with the use of a tapped linear driver is that different sets of LEDs are turned on at different times and for different duration, for example at different interval of a mains cycle. This gives rise to a different light distribution and output intensity for the different sets. Such a tapped linear driver is disclosed in U.S. Pat. No. 8,896,235B1. In this prior art, the sets are turned on in a monotonically accumulative manner namely a first set is turned; then a second set is turned on together with the first set; and then a third set is turned on together with the first and the second set. In this prior art, LEDs in the set that is turned on for longest duration is placed at the center.
Another prior art DE202013000064U1 shows a non-monotonically turning on order wherein the tapped sets in tapped linear driver can be turned on and turned off many time when the voltage increases from zero crossing to the peak. In this prior art, some set is configured to provide more light output to a working plane while some set is configured to provide less light output for indirect lighting.